Conventional photolithography is believed to be limited to about 150 nm in pattern dimensions. While X-ray and ion beam lithography have been demonstrated as viable alternative techniques for creating pattern dimensions below this limit, they are expensive. E-beam lithography has also been proven as a viable technique. However, it is time consuming and, like X-ray and ion beam lithography, expensive. In contrast to such lithographic techniques, imprinting offers an attractive alternative to the fabrication of two-dimensional (2-D) nanometer-scale features, as a result of simpler, faster, and cheaper processing, making this technique a potential replacement for photolithography in mass production.
The above-mentioned lithographic techniques are further limited to fabrication of 2-D and supported features. Imprinting, however, can lend itself to the fabrication of three-dimensional (3-D) features, wherein 3-D features comprise structural variation with depth. Three-dimensional patterning techniques are likely to be important enabling technologies for a number of applications. In microelectronics, for example, the third dimension could possibly allow the speed and memory of microprocessors to go beyond the limitations currently imposed by 2-D features. In optoelectronic industries, 3-D photonic band gap structures are garnering considerable attention because 3-D structures serve to minimize loss of light [Kiriakidis et al., “Fabrication of 2-D and 3-D Photonic Band-Gap Crystal in the GHz and THz Region,” Mater. Phys. Mech., 1:20-26, 2000]. In drug/chemical delivery systems, sensing systems and catalysis, the feasibility of fabricating 3-D structures will provide breakthroughs in the efficiency of controlled delivery, sensing, and selectivity in chemical reactions. For example, a sphere with a meshed surface can be envisioned as a chambered pill that contains multiple drugs or a multifunctional catalysis support.
While 2-D fabrication techniques are mature technology down to the sub-micrometer scale, very little has been reported regarding 3-D sub-micrometer fabrication techniques. Currently, of the limited amount of literature available on sub-micrometer 3-D fabrication techniques, most reports are seen to be mere extensions of various photolithography techniques. For instance, Whitesides et al. have shown that a porous microsphere can be obtained via a self-assembly approach [Huck et al., “Three-Dimensional Mesoscale Self-Assembly,” J. Am. Chem. Soc., 129:8267-8268, 1998], and Yamamoto et al. have demonstrated the fabrication of micrometer scale grooved structures using deep X-ray lithography [Tabata et al., “3D Fabrication by Moving Mask Deep X-ray Lithography with Multiple Stages,” The Fifteenth IEEE International Conference on Micro Electro Mechanical Systems, 180-183, 2002]. Whitesides et al. have also reported on a “membrane folding” method used to create 3-D structures [Brittain et al., “Microorigami: Fabrication of Small Three-Dimensional Metallic Structures,” J. Phys. Chem. B, 105:347-350, 2001]. While most of these techniques have demonstrated the feasibility of creating 3-D sub-micrometer or nanometer scale features, they are not easily implemented for mass production.
Both conventional nano-imprinting [Sun et al., “Multilayer resist methods for nanoimprint lithography on nonflat surfaces,” J. Vac. Sci. Technol. B, 16(6):3922-3925, 1998] and reversal imprinting [Huang et al., “Reversal imprinting by transferring polymer from mold to substrate,” J. Vac. Sci. Technol. B, 20(6):2872-2876, 2002] techniques are attractive alternatives to the above-mentioned techniques in the fabrication of 3-D nano-features, although currently both techniques create 3-D structures through multiple imprinting on patterned substrates or on substrates with topology.
In a known nanoimprint lithography (NIL) process, a thin layer of imprint resist (thermal plastic polymer) is spun coated onto a sample substrate. A mold having predefined topological patterns is brought into contact with the sample and pressed into the polymer coating under certain pressure and at a temperature above the glass transition temperature of the polymer to allow the pattern on the mold to be pressed into the melt polymer film. After being cooled down, the mold is separated from the sample and the pattern resist is left on the substrate. A pattern transfer process, such as reactive ion etching (RIE) is used to transfer the pattern in the resist to the underneath substrate by removal of residue from the substrate.
One known RIE utilizes a gas-plasma to remove the pattern resist from the substrate. A significant disadvantage with this technique is that unless an inorganic mask layer is used to shield the final structures from exposure, the plasma will attack all polymer surfaces, not just the residue layer. This makes adequate preservation of the final structures a difficult task. For example, if the residue layer is too thick, it is necessary to implement a lengthy plasma etch time in order to remove the residue layer. In cases where an inorganic mask layer cannot be used, the dry etching can damage or destroy the final polymer structures. At the very least it would be expected that isotropic etching causes the sidewalls of the final structures to be sloped or tapered rather than vertical.
Additionally, it is known that gas-plasma exposure to most polymeric materials results in chemically modified surfaces, in particular, reactive oxygen plasma often oxidizes the polymer surface. This can be highly undesirable in cases where preservation of the chemical functionality of the polymer structure is important.
Furthermore, depending on the length of the exposure, the physical etching component of an oxygen-plasma RIE etch will roughen the top surface of the final imprinted structures. For most applications, this effect is undesirable. It should be noted that wet-etching has been shown to remove the residue layer, thus avoiding the roughening effect of dry etching, but it is also not selective for the residue layer, which is a pre-requisite for this process.
There is a need to provide an imprint lithographic method of making a polymeric structure, particularly three dimensional micro-sized and nano-sized polymeric structures, that overcome or at least ameliorate one or more of the disadvantages described above.